Iron Enhancement Of Marine Phy Essay Research

Iron Enhancement Of Marine Phy Essay, Research Paper

Iron Enhancement of Marine Phytoplankton

Abstract

As atmospheric carbon dioxide levels increase, efforts have been made to help restore these levels to their natural state. An intriguing hypothesis has been put forward in an attempt to reduce these levels. By seeding regions of the oceans, known as high-nitrate low chlorophyll areas, with iron, it is believed that phytoplankton populations will grow. This in turn, through the process of photosynthesis, will lower atmospheric carbon dioxide levels. Two major experiments that have been conducted in the last decade, examine the reality of such an undertaking. The potential for this method to be used on a global scale appear to be limited at best.

Ever since the Industrial Revolution began in the late 1700 s, the amount of carbon dioxide released into the atmosphere has increased at dramatic rate. The majority of CO2 released into the atmosphere comes from the burning of fossil fuels and from land use such as deforestation, biomass burning and agricultural expansion, which limits the environments ability to recycle CO2 emissions (Walker and Kasting 1992). The concentration of CO2 is expected to double by the middle of the next century, causing global warming to increase by 1.5.C to 4.5.C. Rising global temperatures are expected to raise sea levels, as well as change precipitation levels and other local climate conditions. Changing regional climate could alter forests, crop yields, and water supplies (Rao and Chakravarty 1992). Also, numerous animal and plant species could be in danger of extinction due to an altering of their ecosystems, such as an expansion of deserts into existing range lands (Knox 1999). Due to the wide ranging effects of increased CO2 levels in the atmosphere, many scientists are trying to find solutions that will slow the rate of increasing CO2 levels. One solution that has been suggested is the iron enhancement of productivity in marine phytoplankton as a potential method for the reduction of atmospheric carbon dioxide.

The rich plant life that can be found in our oceans is the major user of CO2 from the atmosphere. It is now believed that the oceans absorb between 30% and 50% of the CO2 released into the atmosphere from the burning of fossil fuels (King et al. 1992). CO2 levels in the atmosphere and dissolved in the ocean s surface layer determine the ocean-water absorption and emission of gas. The amount of CO2 dissolved in water is in turn influenced by the amount of phytoplankton, which consumes CO2 during photosynthesis (LeBorgne and Rodier 1997). Phytoplankters, which are composed of algae and cyanobacteria, are minute single-celled ocean plants that are responsible for approximately 40% of the planet s total annual photosynthetic production and help to reduce atmospheric CO2 levels (Raven 1994). The phytoplankton reduce atmospheric levels by carrying the CO2 they absorb during photosynthesis and transporting them deep into the ocean by the way of dead plants, body parts, and feces (LeBorgne and Rodier 1997).

It might seem that a simple way to reduce atmospheric CO2 levels, would be to increase phytoplankton populations in the oceans of the world. But, there has been some debate as to why phytoplankton populations are not naturally higher than what they currently are. Areas known as high-nitrate, low-chlorophyll (HNLC) regions, are oceanic systems that have low phytoplankton standing stocks, despite high levels of macronutrients (Boyd et al. 1996). These areas include the equatorial Pacific, Southern Ocean and the Subarctic Pacific (Cullen 1991). Some recent theories have suggested the low phytoplankton stocks are the result of grazing by animals further up the food chain, ammonium inhibition of nitrate uptake, mixed layers in excess of the critical depth or, the most popular theory of late, the lack of an essential micronutrient such as iron (Chisholm and Morel 1991).

Due to the extreme insolubility of iron in oxygenated seawater (Moffett and Zika 1987) the potential role of iron as a limiting factor in phytoplankton productivity was appreciated as early as the 1930 s (Harvey 1938). But it wasn t until the clean techniques developed in the 1980 s, was it possible to determine that open-ocean iron concentrations were indeed below the requirement of phytoplankton (Fitzwater et al. 1996).

The next step was to determine if increasing the concentration of iron would indeed increase phytoplankton growth. To test this, a study was conducted using bottles filled with surface waters from the HNLC regions and iron was added to half the bottles and the other half were left alone as controls (Martin et al. 1991). Phytoplankton abundance was monitored in the bottles by various means to see if the addition of iron allowed the phytoplankton to assimilate additional nutrients. The general results were always the same, the total chlorophyll in the iron-enriched bottles were higher than in the control bottles at the end of the experiment, and nitrates were more depleted in the iron-enriched bottles relative to the control bottles (Martin et al. 1991).

The final obstacle in determining the feasibility of using iron enhancement to increase marine phytoplankton productivity to reduce atmospheric CO2 was to conduct an experiment outside of the laboratory, and in the ocean itself. Two large scale expe